1. Field of the Invention
The present invention relates mainly to a battery system and a method for detecting an internal short circuit in a battery system capable of being suitably used as a vehicle power supply device that supplies electric power to an electric motor for driving a vehicle, and in particular to a battery system and a method for detecting an internal short circuit in a battery system that includes a number of battery cells that are connected to each other in series and in parallel to increase output current and battery capacity.
2. Description of the Related Art
In the case where a battery system includes battery cells that are serially connected to each other, the output voltage of the battery system can be high. Also, in the case where a battery system includes battery cells that are connected in parallel to each other, the total amount of output current and the charged capacity can be high. Accordingly, in the case of a battery system that is required to have high output and high capacity for supplying power to an electric motor for driving a vehicle, a number of battery cells are connected to each other in series and in parallel. This type of battery system is discharged and supplies electric power to the electric motor when the electric motor accelerates or drives the vehicle, and is charged by an electric generator or an external charger if the remaining capacity of the battery system is low. In addition, the battery system is charged by the electric generator in vehicle regenerative braking. For the battery system that includes battery cells that are connected in parallel to each other, and has high capacity capable of providing a large amount of current, it is important to uniformly flow currents in the battery cells connected in parallel to each other so that the battery cells operate in uniform states. The reason is that the unbalance among the battery cells may deteriorate a particular battery cell.
A power supply device has been developed that can equalize currents flowing batteries connected to each other in series (Japanese Patent Laid-Open Publication No. 2006-345660).
In the battery system disclosed in JP-2006-345660-A, series battery units each of which includes a number of battery cells serially connected to each other are connected to each other in parallel.
In the battery system, a constant current circuit is connected to each series battery unit to uniformly flow a current in each series battery. Thus, the battery system can uniformly flow currents in the series battery units.
In the battery system, the risk of an internal short circuit will increase in a particular battery cell as a number of battery cells are used longer. Such an internal short circuit may occur in a battery cell in various states. For example, if an internal short circuit occurs caused by a metal fragment that enters a battery cell case, a large amount of short circuit current will flow. Accordingly, it is relatively easily detect such a strong internal short circuit. The reason is that such a strong internal short circuit causes a remarkable voltage drop, or generates heat.
However, in the case where a battery cell is repeatedly charged/discharged, a very small short circuit may occur. In the case where a battery cell is repeatedly charged/discharged, for example, a deposit will deposit on the surface of a positive or negative terminal and may pierce a separator so that a very small short circuit may occur. Since this type of very small short circuit may occur due to a very small deposit, this very small deposit will melt and break due to heat generated by a short circuit current flowing in the deposit. For this reason, such a very small deposit will not continuously create a short circuit. Since this type of very small short circuit will not continuously occur and will create only a very small amount of short circuit current, it is difficult to determine whether a very small short circuit occurs or not.
In addition, in the battery system that includes a plurality of battery cells connected to each other in parallel, it is more difficult to determine whether a very small short circuit occurs in a particular battery cell or not. The reason is that, even when an internal short circuit occurs in one battery cell so that the voltage of the one battery cell drops, a current flows into the one battery cell from other battery cells connected to the one battery cell in parallel so that voltages of the battery cells are equalized.
If a very small internal short circuit repeatedly or continuously occurs, this may finally cause a strong internal short circuit and generate heat, which in turn may bring the battery system in a worse state. In addition, in the case where battery cells are connected to each other in parallel, current will flow into a battery cell in that a very small internal short circuit occurs. This may increase the amount of generated heat. As a result, the battery system may be brought in a still worse state.
From this viewpoint, in the battery system that includes a plurality of battery cells connected to each other in parallel, if it can be determined that an internal short circuit occurs in a battery cell, before the battery system is brought in a worse state, it is possible to restrict the use of the battery system, for example, engine starting is only allowed and the vehicle running by the battery system (i.e., charging/discharging operation) is not allowed. Optionally, “Battery Problem”, “Drive to Service Garage” or the like can be indicated to users. This allows users to more safely use the battery system.
However, the battery system disclosed in JP-2006-345660-A cannot equalize currents in battery cells that compose the series battery unit, and cannot determine whether an internal short circuit occurs in a battery cell or not. For this reason, there is a problem in that safe charging/discharging operation cannot be surely assured if an internal short circuit occurs in a battery cell.
The present invention is aimed at solving the problem. It is an important object of the present invention is provide a battery system, and a method for detecting an internal short circuit in a battery system that includes battery cells that are connected to each other in parallel to increase an output current and a battery capacity, and reliably determines whether an internal short circuit occurs in a battery cell among the battery cells, which are connected to each other in parallel, whereby assuring higher safety.